The ease with which the pH of water is measured obscures the fact that there is presently no clear molecular description for the hydrated proton. The mid-infrared spectrum of bulk aqueous acid, for example, is too diffuse to establish the roles of the putative Eigen (H3O+) and Zundel (H5O2+) ion cores. To expose the local environment of the excess charge, we report how the vibrational spectrum of protonated water clusters evolves in the size range from 2 to 11 water molecules. Signature bands indicating embedded Eigen or Zundel limiting forms are observed in all of the spectra with the exception of the three- and five-membered clusters. These unique species display bands appearing at intermediate energies, reflecting asymmetric solvation of the core ion. Taken together, the data reveal the pronounced spectral impact of subtle changes in the hydration environment.

Predissociation spectra of the H(5)O(2) (+)RG(n)(RG = Ar,Ne) cluster ions are reported in energy regions corresponding to both the OH stretching (3350-3850 cm(-1)) and shared proton (850-1950 cm(-1)) vibrations. The two free OH stretching bands displayed by the Ne complex are quite close to the band origins identified earlier in bare H(5)O(2) (+) [L. I. Yeh, M. Okumura, J. D. Myers, J. M. Price, and Y. T. Lee, J. Chem. Phys. 91, 7319 (1989)], indicating that the symmetrical H(5)O(2) (+) "Zundel" ion remains largely intact in H(5)O(2) (+)Ne. The low-energy spectrum of the Ne complex is simpler than that observed previously for H(5)O(2) (+)Ar, and is dominated by two sharp transitions at 928 and 1047 cm(-1), with a weaker feature at 1763 cm(-1). The H(5)O(2) (+)Ar(n),n = 1-5 spectra generally exhibit complex band structures reflecting solvent-induced symmetry breaking of the Zundel core ion. The extent of solvent perturbation is evaluated with electronic structure calculations, which predict that the rare gas atoms should attach to the spectator OH groups of H(5)O(2) (+) rather than to the shared proton. In the asymmetric complexes, the shared proton resides closer to the more heavily solvated water molecule, leading to redshifts in the rare gas atom-solvated OH stretches and to blueshifts in the shared proton vibrations. The experimental spectra are compared with recent full-dimensional vibrational calculations (diffusion Monte Carlo and multimode/vibrational configuration interaction) on H(5)O(2) (+). These results are consistent with assignment of the strong low-energy bands in the H(5)O(2) (+)Ne spectrum to the vibration of the shared proton mostly along the O-O axis, with the 1763 cm(-1) band traced primarily to the out-of-phase, intramolecular bending vibrations of the two water molecules.

Naturally occurring clay minerals provide a distinctive material for carbon capture and carbon dioxide sequestration. Swelling clay minerals, such as the smectite variety, possess an aluminosilicate structure that is controlled by low-charge layers that readily expand to accommodate water molecules and, potentially, CO2. Recent experimental studies have demonstrated the efficacy of intercalating CO2 in the interlayer of layered clays, but little is known about the molecular mechanisms of the process and the extent of carbon capture as a function of clay charge and structure. A series of molecular dynamics simulations and vibrational analyses have been completed to assess the molecular interactions associated with incorporation of CO2 and H2O in the interlayer of montmorillonite clay and to help validate the models with experimental observation. An accurate and fully flexible set of interatomic potentials for CO2 is developed and combined with Clayff potentials to help evaluate the intercalation mechanism and examine the effect of molecular flexibility on the diffusion rate of CO2 in water.

Molecular dynamics simulations using classical force fields were carried out to study the structural and transport properties of clay mineral−water−CO 2 systems at pressure and temperature relevant to geological carbon storage. The simulations show that the degree of swelling caused by intercalation of CO 2 strongly depends on the initial water content in the interlayer space and that CO 2 intercalation stimulates inner-sphere adsorption of the positively charged interlayer ions on the internal clay surfaces, which modifies the wetting properties of the surfaces. DFT-based molecular dynamics simulations were used to interpret the origin of the observed shift in the asymmetric stretch vibration of CO 2 trapped in montmorillonite. The origin of the shift is attributed to the electric field effects on the CO 2 molecules induced by the water molecules.

Carboxylic acid dimers serve as prototypical systems for modeling the unusual spectral behavior of the hydride stretch fundamental. Large anharmonic effects associated with the pair of cooperatively strengthened OH¯OvC hydrogen bonds produces complicated infrared spectra in which the OH stretch oscillator strength is spread over hundreds of wave numbers, resulting in a complicated band sub-structure. In this work cubic anharmonic constants are computed along internal coordinates associated with the intramolecular OH stretch, intermolecular stretch, and OH bend internal coordinates for the formic acid and benzoic acid dimers. These are then projected onto the normal coordinates to produce mixed states that are used in computing the OH stretch infrared spectrum. For the benzoic acid dimer the calculations accurately reproduce for three deuterated isotopomers the overall breadth and much of the vibrational sub-structure in the observed spectra. For the formic acid dimer, the spectrum is calculated using a model employing a subset of the cubic force constants as well as using the full cubic force field. The spectra calculated for the formic acid dimer are sparser and somewhat more sensitive to the exact positions of the anharmonically coupled states than that of the benzoic acid dimer. Again semiquantitative agreement with experiment is obtained.

Molecular dynamics simulations have been carried out to study decomposition of methane hydrate at different cage occupancies. The decomposition rate is found to depend sensitively on the hydration number. The rate of the destruction of the cages displays Arrhenius behavior, consistent with an activated mechanism. During the simulations, reversible formation of partial water cages around methane molecules in the liquid was observed at the interface at temperatures above the computed hydrate decomposition temperature.

Multiphase Gibbs ensemble Monte Carlo simulations were carried out to compute the free energy of swelling for Na-montmorillonite and Na-beidellite interacting with CO 2 and H 2 O at pressure and temperature conditions relevant for geological storage aquifers. The calculated swelling free energy curves show stable monolayer and bilayer configurations of the interlayer species for Namontmorillonite, while only the monolayer structure is found to be stable for Na-beidellite. The calculations show that CO 2 is intercalated into hydrated clay phases at concentrations greatly exceeding its solubility in bulk water. This suggests that expandable clay minerals are good candidates for storing carbon dioxide in interlayer regions. For Na-beidellite the CO 2 molecule distribution is mainly controlled by the position of the isomorphic substitutions, while for Namontmorillonite the presence of the hydrated sodium ions plays an important role in establishing the CO 2 distribution.

The two-step laser excitation scheme of stimulated emission pumping (SEP) induces shifts of a single water molecule between two remote hydrogen bonding sites on trans-formanilide. This reaction can be initiated by selective excitation of either isomer (CO-bound or NH-bound) with different SEP excitation wavelengths. Energy (E) thresholds for isomerization in both directions have been measured [796 wave numbers NH) CO)